The present invention relates to marine instrumentation of the type which provides information regarding the movement of a marine vehicle through the water. More particularly, the present invention relates to an improved marine vehicle movement transducer and a marine speedometer utilizing the same.
Heretofore, the movement of a marine vehicle, particularly its velocity, has been ascertained by a variety of techniques. Motor powered marine vehicles, sometimes referred to as "speedboats", have typically utilized a pressure sensing tube interconnected with a pressure gauge calibrated to provide a readout of a marine vehicle velocity. The transfer function of this type of marine speedometer is a square root function. Accordingly, the apparatus is very inaccurate at low speed (i.e., 0-15 m.p.h. on a 50 m.p.h. range scale). Furthermore, this type of speedometer is undesirable in that the pressure sensing tube is easily clogged with foreign matter and is subject to damage from water force.
Another technique known in the prior art for detecting the movement of a marine vehicle, to provide for a determination of both velocity and distance logged, utilizes a paddle wheel transducer having a plurality of vanes. One of the vanes of the paddle wheel carries a magnet. The transducer further includes a reed switch which opens and closes in response to movement of the magnet in close proximity thereto. An electronic current averaging circuit connected to the reed switch generates an electrical signal which is proportional to the velocity of the boat. The signal is applied to a suitable electrical meter calibrated to provide a readout of marine vehicle velocity. Due to the limited response capability of magnetically actuated reed switches, marine speedometers utilizing paddle wheel transducers have been restricted to use on sailboats and other low velocity (e.g., up to 15 knots) water vehicles.
As an alternative to a magnetically actuated reed switch sensor, other paddle wheel type transducers have utilized an electrical pickup coil to sense the passage of magnet mounted in the paddle wheel vanes. Rotation of the paddle wheel results in the development of an electrical voltage in the coil which is applied to a meter calibrated to indicate the speed of the boat. Again, however, due to the limited response capability of the coil sensor, such transducers can be effectively utilized only on low velocity water vehicles.
It is a further disadvantage of magnetic paddle wheel transducers that the additional mass of a magnet carried on a single paddle wheel vanes creates a rotational eccentricity which results in a friction loss on rotation of the paddle wheel about its mounting.